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RE: spectral response acceleration definition
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- Subject: RE: spectral response acceleration definition
- From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
- Date: Fri, 6 Jan 2006 17:34:55 -0500
I agree with A. Daneshvar's response and will add some
additional explanation.
I would say that "spectral" has more to do with variation
in response based on the natural period of the structure. The response
spectrum curve represents how a single degree of freedom system with any given
natural period would respond to a given ground motion. The short period
spectral response represents the peak response or maximum response (i.e., due to
resonance of the structure natural period and the applied earthquake). The
one second spectral response simply represents a point on the curve in the
long period range of response. With these two points defined, a full
response spectrum curve can be constructed to cover all periods, using the code
defined equations.
William C. Sherman, PE From: daneshvar.ahmad [mailto:daneshvar(--nospam--at)pidec.com] Sent: Tuesday, January 03, 2006 11:02 PM To: seaint(--nospam--at)seaint.org Subject: RE: spectral response acceleration definition Dear
Bruce, Hi, I believe the first
and third paragraph is completely right, But there is a misunderstanding in the
second one. My comment is as follow: We need short (0.2 sec)
and long (1.0 sec) period “spectral response acceleration” to construct
“response spectrum curve” accordingly. Short period spectral response
acceleration Ss is similar to Ca in 97 UBC.( as a measure of constant
acceleration part of response spectrum curve for relatively rigid structures
with short period), and long period spectral acceleration S1 is similar to Cv in
97 UBC (as a measure of constant velocity part of response spectrum curve for
relatively flexible structures with long period). Therefore, after constructing
of response spectrum curve the spectral response acceleration for other periods
to be extracted from this curve. Code permits to evaluate the Ss and S1 for
location between the contours of the maximum considered earthquake ground
motion’s maps by interpolation. Have a good
presentation! Regards, From: Bruce
Holcomb [mailto:bholcomb(--nospam--at)brpae.com] I have been having trouble with the term “spectral” in
“spectral response acceleration”, so I’ve been doing some studying. Can
someone verify my findings below? Or set me straight? I’m supposed
to give a presentation at my office and I want to make sure I have it straight
before I try to explain to others. The “spectral response
acceleration” plotted in the “Maximum Considered Earthquake Ground Motion” maps
in the IBC is an acceleration with an annual probability of exceedance of 2% in
50 years (return period of 2500 years) and is for a site class “B” soil.
The spectral response acceleration is derived from many possible earthquakes of
various magnitudes at various distances from the
site. The spectral response
acceleration is plotted for period of 0.2 seconds and 1.0 seconds because these
two periods encompass most building periods. Periods between these values
(and to a certain extent beyond these values) can be interpolated
(extrapolated). The reason the “design
spectral response acceleration” is 2/3 of the “site adjusted spectral response
acceleration” is that the lower bound estimate of the margin against collapse is
a factor of 1.5 (approximate… judged by experience). Therefore, the
“design” earthquake ground motion is 1/1.5 (2/3) of the maximum considered
earthquake ground motion because we want to prevent collapse, but the
construction cost would be too great, if we designed to prevent all damage to a
structure. Bruce |
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